The manipulation of individual submicron-sized objects has been the focus of significant efforts over the last few years. Specifically, efforts have been made toward developing touchless electromagnetic positioning systems for micrometer- and nanometer-sized particles. As opposed to micromanipulators and atomic force microscopy, touchless technologies represent a different approach to the application of controlled forces to small particles while simultaneously removing the difficulties of other approaches such as, for example, stiction. Developments in lab-on-a-chip technologies, heterogeneous integration of electronic components, and studies in nanofluidics are among many which are driven, in large part, by the rate of innovation in their tools, and these technologies and others would derive significant benefit from developments in positioning systems for micrometer- and nanometer-sized particles.
Various tools have emerged to fill this need. The most well known are optical tweezers which use a finely focused laser as the electromagnetic source to allow the arbitrary positioning of very small objects. Optical tweezers have the ability to specify a location in space by focusing a laser beam. Positively polarizable particles are attracted to this region. This is very useful for manipulating positively polarizable objects. Since the beam represents a region of attraction, there is no need for negative feedback.
Magnetic tweezers have recently been reported in which a magnetic particle is controlled using the magnetic field generated by an array of coils. See, for example, C. Gosse and V. Croquette, Biophys J 82, 3314-3329 (2002), the disclosure of which is incorporated herein by reference. While optical tweezers allows for such manipulation, resolution is limited due to physical constraints on laser spot size. The beam is very localized and the force that is applied is indirect. That is, when the particle drifts to the outer edge of the focus, it feels an attraction back, but quantifying the applied force is difficult. Additionally atomic force microscopy can be used to position nanoscale particles but involves a complex apparatus. Magnetic tweezers also allow arbitrary positioning of particles, but this approach is constrained to magnetic entities.
Electric fields have remained very attractive because of the ease with which they are generated on very small scales with microelectrode structures. Many electrokinetic techniques have emerged such as positive dielectrophoresis, negative dielectrophoresis, traveling wave dielectrophoresis, and electrorotation. While particles have been manipulated within the space between electrodes, it has not been done arbitrarily. Unlike the arbitrary positioning of optical and magnetic tweezers, these techniques have only been used to direct particles toward or away from electrodes. The use of electrodes in this fashion “discretizes” the affected space so that the particles can only be controlled with a spatial resolution similar to that of the electrode array itself. These techniques are thus limited to accuracies defined by the electrode spacings themselves.